TURBO PASCAL CORNER ............................................... Rick Ryall
For those of you who program in Pascal, this column will be dedicated to
sharing some things that I have discovered in the course of using Borland's
excellent implementation of that language. I will try to cover both CP/M and
MS-DOS versions, to keep almost everybody happy.
This month's installment deals with avoiding the problem that occurs when
an attempt is made to send a character to a printer that is not connected to
the computer, which hangs up the system until the printer is connected or the
computer is reset. It would be nice to test the printer port before sending a
character out to see if the printer is ready to receive one. Fortunately,
there is a way to do just this.
FUNCTION PrinterReady : boolean; FUNCTION PrinterReady : boolean;
type RegisterRecord = const
record case integer of Status = 14;
1:(AX,BX,CX,DX,BP,SI,DI,ES,Flags:integer); begin { PrinterReady, CP/M version }
2:(AL,AH,BL,BH,CL,CH,DL,DH: byte); PrinterReady:= ( Bios(Status)=255 );
end; end; { PrinterReady }
var
Status : byte;
Registers : RegisterRecord;
begin { PrinterReady, MS-DOS version }
{ initialize registers }
fillchar( Registers,sizeof( Registers ),0 );
with Registers do
begin
AH:= $01; { code for reset printer }
DL:= $00; { printer number, 0= COM1 }
intr( $17, Registers ); { reset printer port }
AH:= $02; { code for get printer status }
DL:= $00; { printer number }
intr( $17, Registers ); { get printer status }
Status:= AH;
end;
PrinterReady:= not Odd( Status shr 4 );
end; { PrinterReady }
The CP/M version is fairly straightforward and shouldn't require too much
explanation.
There are two things worth noting in the MS-DOS example above: 1). The
printer port must be reset before the status byte is returned correctly (they
don't tell you this in the reference manuals) 2). The type declaration could
be a global declaration instead of a local one, since the register type is
used for all interrupts and function calls in the MS-DOS version of Turbo
Pascal.
Below is an example of how this function might be used to prevent hanging
up a program if someone has neglected to connect or turn on their printer.
That's it for this month.
PROCEDURE CheckPrinterStatus;
const
Return = ^M;
ESC = #27;
var
Key : char;
Interrupted : boolean;
begin
Interrupted:= false;
while not( PrinterReady or Interrupted ) do
begin
gotoXY(1,3);
write( 'Please check your printer and press RETURN to continue, or ESC to exit' );
ClrEol;
repeat
read( Kbd, Key );
if ( Key = ESC ) and not KeyPressed then Interrupted:= true;
{ not KeyPressed is required for MS-DOS only }
until ( Key = Return ) or Interrupted;
end;
end; { CheckPrinterStatus }